The present invention relates to a charged particle beam system, such as an electron microscope, an electron beam lithography apparatus, and a focused ion beam system, more specifically, to an extremely high degree of vacuum technology of an electron gun or ion gun part.
Conventional scanning electron microscope (SEM) and electron beam writer (EB) each accelerate an electron beam emitted from an electron gun constructed with a field emission type or thermal field emission type electron source, convert it into a narrow electron beam with electron lenses, scan it on a sample as a primary electron beam using a scanning deflector, and perform (in the case of the SEM) detecting secondary electrons or reflected electrons obtainable from the above processes to render an image, or (in the case of the EB) drawing patterns registered in advance on a resist film coated on the sample. As a material of the electron source, in the case of the multi purpose SEM, tungsten is used; for the electron source for inspection of patterns of semiconductors, there is a case where tungsten is made to contain zirconia. Furthermore, in the case of the EB, there may be a case of using LaB6.
In order to make the above-mentioned electron source emit an excellent electron beam current for a long period, it is necessary to maintain an ultra-high vacuum (10−7 to 10−8 Pa) in surroundings of the electron source. For this reason, in the conventional system/apparatus, a method for evacuating the surroundings of the electron gun by ion pumps to construct a differential pressure system is taken. Moreover, there is a charged particle beam system (for example, see U.S. Pat. No. 4,833,362) in which by building a non-evaporable getter pump in the system, the system is made to be able to obtain a higher degree of vacuum.
The non-evaporable getter pump is a pump for chemically absorbing and fixing gas molecules on its surface, and can maintain an evacuation function without supplying energy at all if heating once activates it. With increasing coverage of the alloy surface with gas molecules, a pumping velocity decreases. However, this pump has a feature that if the alloy is reactivated by being heated again, gas molecules adsorbed on the surface diffuse into the inside of the alloy, and fixed eternally, whereby a pure surface is exposed and returns to a state of capable of adsorbing the gas again. In many cases, such metals are hydrogen absorbing alloys, and are used also as pumps for exhausting hydrogen. As a technique of using hydrogen exhaust performance effectively for a long time, a technique is known whereby a hydrogen absorbing alloy is isolated by a valve and the hydrogen absorbing alloy is made to adsorb the gas after exhausting gasses other than hydrogen.
However, since the non-evaporable getter pump has a drawback of being less able to exhaust rare gasses, such as argon gas and helium gas, and gasses which are stable electrochemically, such as methane, the techniques has not been yet in the actual use.
In order to solve this problem, there is a method making combined use of the ion pump and the non-evaporable getter pump (for example, see Japanese Patent Application Laid-Open No. 2006-294481). The ion pump tends to have a smaller pumping velocity and reduced exhaust efficiency when the degree of vacuum becomes higher while the non-evaporable getter pump has a feature that it can maintain high pumping velocity regardless of the degree of vacuum. On the other hand, combined use of the ion pump and the non-evaporable getter pump is effective because it can demonstrate a characteristic which has good points of the ion pump and the non-evaporable getter pump, by compensating a drawback of the non-evaporable getter pump that the rare gasses, such as argon gas and helium gas, described above and electrochemically equilibrated gasses, such as methane, are hard to exhaust with the ion pump.